Solo or single-cam compound bows are well known in the art. A plurality of these have been previously disclosed, some of which are described in U.S. Pat. Nos. 5,505,185 and 5,368,006, each of said patents being incorporated by reference as if fully set forth herein. Dual-cam compound bows are also well known in the art, some of which are described in U.S. Pat. Nos. 4,739,744 and 5,040,520, each of said patents being incorporated by reference as if fully set forth herein, and in which are described in detail the mechanics of a compound bow including non-circular dual cam members which impart dynamic forces on the bow limbs.
Whether single-cam or dual-cam, the purposes and advantages of compound bows are well known to those skilled in the art and need not be repeated herein. Compound bows typically comprise: a handle from which resilient bow limbs extend oppositely; pulley means comprising cams, levers, and/or pulleys and typically being disposed at the tips of the limbs of the bow; and one or more cables coupled to the bow limbs and/or pulley means to give assorted mechanical advantages. One important characteristic of compound bows is the "let-off" (i.e., an abrupt decrease) of the draw force at the end of the draw, thereby allowing an archer to more readily maintain the bow in a fully drawn position while accurately aiming the arrow and/or waiting for game to pass within shot.
When a bow is drawn potential energy is stored in the bow limbs which are deflected substantially equally when the bow string (or draw cable) is drawn. Stored potential energy may be calculated from the draw force as a function of draw distance. Potential energy is converted to kinetic energy when the archer releases the drawn bow with the arrow placed on the draw cable, thereby allowing the bow limbs to return to their resting position and propelling the arrow. Kinetic energy may be calculated from the speed and mass of the arrow, which may in turn allow the efficiency (i.e., the fraction of potential energy converted to kinetic energy) of the bow to be calculated. It is well accepted in the industry and within the sport that efficiency is critical to bow performance: the more efficient a bow the faster the bow will propel an arrow of given weight for a given draw length and given peak draw force. Previous compound bows, single- or dual-cam, have AMO standard efficiencies of approximately 70-81% (see Table 2 on page 80 of "Bowhunting World, August, 1995, which is incorporated by reference as though fully set forth herein; see Table 2 on page 68 of "Bowhunting World" December, 1996, which is incorporated by reference as if fully set forth herein; see FIG. 3 on page 62 of "Bowhunting World" April, 1997, which is incorporated by reference as if fully set forth herein; and see Table 2, Bowhunting Buyers Guide, 1997, which is incorporated by reference as if fully set forth herein). A number of bow characteristics impact the efficiency of a bow, but many of these are well known in the art and therefore need not be repeated herein.
However, the advantages of let-off and energy storage come at the price of a relatively complicated set of cable and pulleys as part of the bow. One particular problem caused by this complex system is that of "nock travel". (The nock point is the point on the draw cable where the end of the arrow is fitted for shooting.) Ideally, the nock point should move straight back as the bow is drawn. Any deviation of the nock point upward or downward as the bow is drawn is called nock travel, and is highly undesirable. Nock travel may affect the accuracy of the bow, the efficiency of the bow, or both.
It is therefore generally desirable to provide a compound archery bow having desirable performance characteristics, which may include but are not limited to: peak draw weight, draw length, draw weight profile, stored energy, efficiency, arrow speed, let-off, recoil, vibration, noise, and/or nock travel. In particular, a compound bow having a force-multiplying middle pulley may improve peak draw weight, draw length, draw weight profile, stored energy, efficiency, arrow speed, and/or let-off characteristics of the bow while reducing and/or eliminating nock travel.